Disinfection  of  potable  water  is  the  specialized 
  treatment  for  destruction  or  removal  of  organisms 
  capable of causing disease. 
  Disinfectants  widely  used  in  water  treatment  are 
  oxidizing agents such as chlorine. 
  Being  a  strong  oxidant  chlorine  will  react  with  the 
  oxidizable  materials  present  in  water  before  it  has  a 
  chance to act as a disinfectant.  
   Chlorine in water:
   Chlorine has been widely used in disinfecting water. It is 
   cheap reliable and easy to handle. 
   Chlorine and water react according to the following 
   equation.
   Cl  + H O                       HOCl (hypochlorous acid)+HCl
    2   2
   HOCl                           H+ +  OCl –  (hypochlorite ion)
  Hypochlorous acid is a strong disinfectant because of its 
  ability  to  diffuse  easily  through  the  cell  walls  of  a 
  microorganisms and disrupt the life function. 
  The relative concentration of HOCl and OCl –   vary with pH, 
  water  temperature  and  the  concentration  of  chlorine  in 
  solution. 
  HOCl  is many times stronger as an oxidant than OCl – . The 
  predominant concentration of HOCl and OCl –   are below 
  pH=6  and  above  pH=7.5  respectively.  Therefor,  the 
  disinfection power of chlorine decreases with increase in 
  pH. 
    The  chlorine  existing  in  water  as  hypochlorius  acid, 
    hypochlorite ion and molecular chlorine is defined as 
    free available chlorine.
    Free available chlorine react with ammonia and many 
    organic amines to form chloramine which is less active. 
    This  chloramine  is  known  as  combined  available 
    chlorine. 
    NH + HOCl                           NH  Cl (monochloramine) + H O
       3                        2                        2
    NHCl+ HOCl                         NHCl  (dichloramine)         + H O
       2                          2                      2
    NHCl  + HOCl                         NCl  (trichloramine)          + H O
         2                       3                       2
  Break point Chlorination:
  When chlorination is added to water, it is consumed in 
  oxidizing  a wide variety of compounds present in water 
  (organic matters and other compounds). No chlorine can 
  be  measured  until  the  initial  chlorine  demand  is 
  satisfied (point A – B) In the following figure. 
  After  meeting  the  initial  demand,  the  added  chlorine 
  reacts with the ammonia to produce combined chlorine 
  residual  (chloramines) which increases with additional 
  dosage until a maximum combined residual is reached 
  (Point B – C).